Many sound reinforcement applications require the accurate reproduction of live or recorded program material that has a wide frequency range, typically 40-18,000 Hz. Yet no single transducer practical for use in the art is capable of both accurately and efficiently reproducing this range of frequencies at high power levels. As a result, virtually all sound reinforcement systems electronically divide the program material into two to five separate frequency bands and provide a separate transducer subsystem for each band, optimized for the reproduction of its range of frequencies.
This use of plural subsystems for the reproduction of full range program material has many associated disadvantages including not just its impact on the total size, weight, cost and complexity of the system, but losses in the fidelity of reproduction produced by discrepancies between the various transducer subsystems in dispersion, transient response, projection, phase/time alignment, and tonal quality. The aural disadvantages of multiple transducer use are particularly important at higher frequencies where shorter wavelengths increase both the incidence and severity of phase cancellation effects between transducers operating in adjacent frequency bands as well as between transducers operating in the same frequency band where their dispersions overlap.
It has therefore, long been an object to reproduce full range program material using the minimum practical number of transducer subsystems, and, particularly in the high frequency region, with the minimum practical number of transducers. As a result, efforts have long been devoted to methods of increasing both the useable frequency range of high frequency components and their power handling ability.
Basic physical factors have, however, seemed to render impractical the combination of extended high frequency response and the high power handling ability required to achieve the object of minimizing both the number of transducers and of transducer types required for the high frequency portion of a sound reinforcement system. The decreasing wavelengths of higher frequencies place a premium on transducers whose active surface area (diaphragm) is small, and as such, capable of rapid acceleration and deceleration by the electromechanical motor of the transducer. Accordingly, a "1-inch" compression driver, such as the Electro-Voice DH2305 is capable of the desired extended high frequency response. Conversely, the limited size of the elements of the transducer also place limits on its power-handling capability, limits which render the "1-inch" driver inadequate for most high-level sound reinforcement applications.
Obviously, the size of the transducer can be scaled up to increase its power handling capability. "2-inch" compression drivers are readily available, and their increased size does indeed increase power handling ability, but not without tradeoffs. Increasing the size and mass of the active area of the transducer/diaphragm also serves to reduce the efficiency with it can reproduce higher frequencies. As a result (as illustrated in FIG. 1), the improvement in power handling is largely offset by a marked reduction in high-frequency response. If the "2-inch" driver is employed as the sole high-frequency transducer, then extended range high-frequency reproduction can only be maintained by electronically decreasing the efficiency of the driver at lower frequencies through frequency-selective attenuation of the program material such that the transducer subsystem as a whole displays a more consistent frequency response. This, however, sets a limit on the power-handling ability of the driver which offsets much of the benefit of the increase in size. Further, the increased size and mass of the diaphragm reduces the transient response of the driver, which is particularly noticeable at higher frequencies. These effects many be reduced by the use of exotic diaphragm materials but only at a very substantial increase in per unit cost and decrease in reliability which has priced drivers employing such materials out of reach of most users.
Alternatively, the high frequency region may be further divided, the lower portion of the band (typically that below 6-7 kHz) reproduced by the "2-inch" compression driver, and the upper portion by a specialized high frequency driver, the "super tweeter". This allows the compression driver to operate in the most efficient portion of its range and hence increases its power handling ability, but results in the various practical and aural disadvantages of multiple transducer subsystem use previously described.
If, therefore, 1-inch compression drivers are inherently the optimal driver for extended range high-frequency reproduction, then some method of bringing a plurality of them to bear on a common axis is required for accurate reproduction of full range program material in high-power sound reinforcement applications.
In such applications, it would be obvious to simply employ a plurality of such compression drivers and their associated horns aimed along a common axis, but phase-cancellation effects between the pressure waves of the plurality of radiators are severe enough to render this approach less than satisfactory.
Alternatively, an apparatus may be used to couple a plurality of such drivers to the throat of a common horn. This has the benefit of producing a single source and would therefore be expected to prevent the phase-cancellation effects of plural radiators, but prior art apparatus of this type has only succeeded in localizing the phase-cancellation effects within the multiple-driver adaptor itself. The benefits of such adaptors have thus been offset by an increase in distortion product such that, despite their potential advantages, few full-range professional sound reinforcement systems employ them.
The potential advantages of such a multiple-driver approach to high-frequency sound reproduction are, nonetheless, so substantial that designers of sound reinforcement equipment have, over the last half century, devoted considerable attention to the development of a multiple-driver adaptor which minimizes distortion product.
Such adaptors comprise a plurality of tubular passages, each coupled on one end to the driver and on the other to an open chamber equal in diameter to the throat of the horn flare to which the adaptor is coupled. In certain embodiments, illustrated by adaptor 30 of FIG. 2, the route of these passages 8 and 8A has been curved. It has been found, however, that lower distortion results if the passages are straight.
Refer now to FIG. 3, a crosssection of a multiple-driver adaptor 33 which represents the best teaching of prior art with respect to proper design of such adaptors. Drivers 1 and 1A are standard 1-inch compression drivers coupled to passages 8 and 8A which are straight. The angle of the passages 8 and 8A have been adjusted with respect to the central axis of the horn flare 2 such that a projection 8P of the wall of the passage will not intersect the wall of the horn flare 2. These features have a significant effect on distortion product, but when applied by the applicants had failed to produce sonic quality improved over that of a single 2-inch driver.
It therefore remains an object to produce a method of coupling multiple compression drivers to the throat of a common horn flare without producing levels of distortion product which offset the potential benefits of such an arrangement.